Direct Action by Hubris

For a puzzle whose only theme was an entry method to be discovered, this
was remarkably challenging. The extra words in the clues were by no means
easy to spot, and even when found they initially didn't seem to help much.
Until the rule governing grid entry could be determined, cross-checking letters
were of little help, so the solver was left with the task of solving most
clues blind. Eventually, for Gregson, the first letters of the extra words
seemed to spell out "Grid entries must conform literally...", which still
wasn't much help.

Finally, realising that another pass through the extra words might yield
a more informative message, he found that the fourth letters in sequence
completed the rule thus: Grid entries must conform literally to their direction,
i.e. north to south, or west to east, or vice versa". Thus was revealed that
entries could be reversed and that the direction of each entry would give
a transformation such as "N to S", meaning that a north-to-south entry would
get each N replaced with S. Once this rule was established it was a relatively
straightforward matter to fill in the grid.